The present invention relates to an output circuit for a video signal recording and reproducing apparatus such as a video tape recorder, and more particularly to a circuit for converting a signal reproduced from a recorded video signal to a signal in the form similar to a television broadcast wave.
In a video tape recorder or a video disk player or the like, a television receiver is commonly used to display a signal reproduced from a recording medium on which video signals have been recorded. Home-use television receivers are designed to receive the video signal in the form of modulated VHF or UHF carrier wave, and very few of them are provided with terminals to which the video signal can be directly applied. Accordingly, in the home-use video signal reproducing apparatus such as home-use video tape recorder, means for producing the reproduced output signal in the form of a modulated signal including a modulated VHF or UHF carrier wave, like the television broadcast wave, such as an RF converter is required. For a carrier frequency for the RF signal in the RF converter, two carrier frequencies of two adjacent television channels having successive frequencies are usually taken up, and one of them is selected by a user. As is well known, since the broadcast waves are not assigned to two channels having successive frequencies in a given area according to a channel plan for the television broadcast, the non-assigned vacant channel is utilized to supply the reproduced signal from the video tape recorder or the like to the television receiver so that the television receiver which receives the broadcast wave is not disturbed.
FIG. 1 shows a block diagram illustrating an example of a conventional RF converter. In FIG. 1, a video signal from the video signal reproducing apparatus such as video tape recorder is applied to a terminal 1. The video signal is then applied to a modulator circuit 2 where it is converted to a modulated signal capable of being received by a conventional television receiver. Numerals 3 and 4 denote carrier generator circuits which produce carrier waves of two adjacent television channels having successive channel frequencies. A user operates a switch 8 to select one of the carriers from the carrier generators 3 and 4 which is to be supplied to the modulator circuit 2. Assuming that the carrier generator circuits 3 and 4 produce the carrier frequencies f.sub.r1 and f.sub.r2, respectively, the output signal of the modulator circuit 2 has frequency spectra shown by A and B in FIG. 2. Since this signal has upper and lower sideband waves symmetrically around the carrier, it is necessary to eliminate the lower sideband wave by a filter so that the signal meets the vestigial sideband transmission system television broadcast standard. That is, for the carries f.sub. r1 and f.sub.r2, the bands of the output signals must be within frequency ranges W.sub.a and W.sub.b, respectively, shown in FIG. 2. Filters 5 and 6 shown in FIG. 1 are bandpass filters for that purpose. The filter 5 functions to pass frequency band W.sub.a for one of the channels while the filter 6 functions to pass frequency channel band W.sub.b for the other channel. Switches 9 and 10 are ganged with the switch 8 so that the passband is selected in accordance with the carrier frequency selected. In this manner, the modulated signal capable of being received by the conventional television receiver is produced at a terminal 7.
In such a circuit arrangement, many switching circuits for the RF signal are necessary. Since the signal having frequency components in the VHF band or higher ones must pass through these switching circuits, it is necessary to use the switches designed to have excellent high frequency characteristics. Such switches are usually expensive and the use of these switches in great number results in the rise of cost. Therefore, such a circuit arrangement is undersirable for the home-use equipment.